In an effort to protect vehicle operators and occupants, vehicle manufacturers and their suppliers are increasingly designing and manufacturing vehicles with additional and improved safety features. One such safety feature that has been incorporated into a number of vehicle types is anti-lock braking, which can take a variety of forms.
Vehicle anti-lock brake systems are designed to maximize the ability of a vehicle operator to bring a vehicle to a controlled stop on any type of road surface. The system accomplishes this goal by preventing the vehicle brakes from prematurely halting vehicle wheel rotation, or "locking" the vehicle wheels, regardless of the road surface and the pressure applied to the brake pedal by the vehicle operator.
Typical vehicle anti-lock brake systems comprise a vehicle wheel speed sensor for providing input to an anti-lock brake system control unit. The control unit controls an anti-lock brake system control valve interposed between the brake master cylinder and the individual wheel brakes of an hydraulic brake circuit. The control valve, in turn, regulates hydraulic brake fluid pressure in the individual wheel brakes to implement anti-lock braking.
In operation, the vehicle wheel speed sensor not only measures the vehicle wheel speed, but also provides input to the control unit for determining a vehicle speed. The control unit monitors the vehicle and vehicle wheel speeds for an indication of an anti-lock braking event. First, based upon the vehicle speed, the control unit typically determines a slip threshold. Using the vehicle velocity as a reference, slip threshold may be expressed as the difference between a selected velocity and the vehicle velocity.
Next, the control unit compares the vehicle wheel velocity to the vehicle velocity to determine a departure depth. Again using the vehicle velocity as a reference, departure depth may be expressed as the difference between the vehicle velocity and the wheel velocity. During normal vehicle braking, the wheel velocity closely matches the vehicle velocity. Thus, during normal vehicle braking, the difference between the vehicle velocity and the wheel velocity is nominal.
However, during an anti-lock braking event, the wheel velocity decreases significantly below, or "departs" from, the vehicle reference velocity. In such a situation, as for example during hard braking on an ice covered road, the frictional force between the vehicle brake pads and the vehicle wheel exceeds that between the vehicle wheel and the road surface. Uncontrolled, such a frictional force differential causes the vehicle wheel to cease rotating, or to "lock."
In turn, locking causes the vehicle wheels to slip or "skid" rather than roll, over the road surface. Such vehicle wheel skidding dramatically reduces traction and the ability of the vehicle operator to bring the vehicle to a controlled stop.
To prevent such vehicle wheel lock and the accompanying problems, the control unit of an anti-lock brake system activates the anti-lock brake system control valve to regulate hydraulic brake fluid pressure in the individual wheel brakes during an anti-lock braking event. More specifically, the control unit compares the departure depth to the slip threshold and actuates the control valve when the departure depth exceeds the slip threshold in order to isolate the individual vehicle wheel brakes in the hydraulic brake circuit from the master cylinder, thereby halting any increase in brake fluid pressure in the vehicle wheel brakes and preventing incipient vehicle wheel lock.
More particularly, when, during vehicle braking, the departure depth exceeds the slip threshold, the control valve isolates brake fluid in the individual wheel brake from the increasing brake fluid pressure in the master cylinder in order to hold brake fluid pressure in the wheel brake constant. If the isolated brake fluid pressure in the wheel brake is still high enough to cause incipient wheel lock, the anti-lock brake system then bleeds, or dumps, brake fluid from the wheel brake to reduce brake fluid pressure therein.
Thereafter, the anti-lock brake system typically holds brake fluid pressure in the wheel brake constant until such time as the departure depth no longer exceeds the slip threshold, indicating that the vehicle wheel is again travelling at or near the velocity of the vehicle. At that time, the anti-lock brake system then increases, or builds, brake fluid pressure in the wheel brake by reapplying brake fluid thereto. Reapplication of brake fluid to the wheel brake may be at a steep or gradual rate, or some combination thereof, depending upon the circumstances or the control desired.
It should be noted that to maintain smooth braking and optimum vehicle control, some reapplication of brake fluid to the wheel brakes must be undertaken where the isolation of the brake fluid in the wheel brakes from that in the master cylinder has been prolonged, for example on the order of one hundred milliseconds or greater. Such reapplication must be undertaken in order to raise brake fluid pressure in the wheel brake to a level approximating that in the master cylinder before the control valve may de-isolate the wheel brake from the master cylinder.
One problem associated with anti-lock brake systems as described above is their tendency toward false, or premature, activation. Premature activation can occur in a number of circumstances, such as where a road surface is partially ice covered or bumpy and individual vehicle wheels experience different coefficients of friction during braking. Typical anti-lock brake systems activate when the departure depth of any one of the vehicle wheels exceeds the slip threshold, despite the fact that the wheel would not have experienced excessive slip.
Moreover, typical anti-lock brake systems decrease the slip threshold after activation in order to increase anti-lock brake system sensitivity during an anti-lock braking event. Under normal conditions, this action improves the ability of the vehicle operator to slow or stop the vehicle in a controlled fashion. However, in the event of premature activation of the anti-lock brake system, such action merely compounds the problem of premature activation.
Previous attempts to solve the premature activation problem included controlling the anti-lock brake system based on the assumption that if the control valve isolated the wheel brake from the master cylinder and the anti-lock brake system did not undertake a reduction of brake fluid pressure in the wheel brake, then the anti-lock brake system had activated prematurely. However, such a technique has proven unsatisfactory in that it is possible to meet the pressure reduction criterion while still encountering premature activation of the anti-lock brake system.